Streptomycin and Spectinomycin resistance mediated by plasmids.

Resistance to tetracycline (Tc), chloramphenicol (Cm), streptomycin (Sm), and sulfanilamide (Sa) was surveyed in clinical isolates of Escherichia coli and Shigella strains. Among the Sm Sa-resistant strains, the frequency of nonconjugative r (Sm Sa) plasmids was much higher than that of conjugative R plasmids encoding double resistance. The biochemical mechanism of Sm resistance mediated by the conjugative plasmids R(Tc Cm Sm Sa) and R(Cm Sm Sa) and about half the numbers of conjugative R(Tc Sm Sa) and R(Sm Sa) plasmids tested were found to be due to adenylylation of the drug. The remaining conjugative R(Tc Sm Sa) and R(Sm Sa) plasmids and all nonconjugative (Sm Sa) plasmids tested inactivated Sm by phosphorylation.